Method for Coating a Blade and Blade of a Gas Turbine

ABSTRACT

A method is provided for coating a hollow, internally cooled blade ( 1 ) of a gas turbine, in which method an outer coating ( 5 ) comprising an MCrAlY-based bonding layer ( 6 ) and a ceramic thermal barrier layer ( 9 ) of zirconium oxide is applied to the base material ( 3 ) of the blade ( 1 ) on the outer side of the blade ( 1 ) and an inner coating ( 4 ) comprising a Cr diffusion layer ( 7 ) is applied to the base material ( 3 ) of the blade ( 1 ) on the inner side of the blade ( 1 ). The MCrAlY-based bonding layer ( 6 ) is thereby applied to the finished blade ( 1 ). At the same time, along with the inner coating ( 4 ), the Cr diffusion layer ( 7 ) is also applied to the MCrAlY-based bonding layer ( 6 ) of the outer coating ( 5 ) by chemical vapour deposition. Subsequently, an Al diffusion layer ( 8 ) and an outer brittle Al build-up layer are applied by chemical vapour deposition to the bonding layer ( 6 ) coated with the Cr diffusion layer ( 7 ). After that, the outer brittle Al build-up layer is removed by an abrasive treatment and the ceramic thermal barrier layer ( 9 ) is applied to the Al diffusion layer ( 8 ).

The invention pertains to a method for coating a hollow, cooled blade of a gas turbine with the features of the introductory clause of Claim 1 and a blade of this type for a gas turbine.

In modern gas turbines, almost all of the surfaces in the hot-gas section are provided with coatings to protect them from high-temperature oxidation and high-temperature corrosion. Exceptions in many cases are still the turbine blades in the rear of an array. Chromium oxide-forming and aluminum oxide-forming diffusion layers and cladding layers have become widely adopted. In the latter case, MCrAlY cladding layers (M═Ni, Co) are the most widely used in stationary gas turbines.

In parallel, cooled blades, that is, hollow blades, are being produced to an increasing extent to keep the material temperature at a level which the available materials can withstand. Cooling is usually accomplished with compressed air, which, depending on the pressure in the environment of the blades to be cooled, is taken from certain stages of the compressor of the gas turbine system. The temperature of the cooling air is between approximately 450° C. at the inlet to the blade and approximately 800° C. at the outlet from the blade. In the case of highly stressed cooled blades, internal coatings are being used to an increasing extent to prevent oxidation-caused attacks on the grain boundaries, which can have the effect of initiating cracks. Through the use of internal coatings in blades, the thermomechanical fatigue (TMF) life can be considerably extended.

For reasons of process technology, therefore, Al diffusion layers produced by alitization have been used so far almost exclusively. The disadvantage of an Al diffusion layer as in internal coating is to be found in the brittleness of the Al diffusion layer and its relatively low resistance to the sources of high-temperature corrosion, which can be present as contaminants in the cooling air. It has also been pointed out that Cr diffusion layers produced by chromizing can be used to protect against high-temperature oxidation and corrosion.

Heat insulating layer systems are used on cooled components such as the blades of gas turbines. Heat-insulating layer systems in gas turbines always consist of a metallic bonding layer diffusion bonded to the base material, on top of which a ceramic layer with poor thermal conductivity is applied, which represents the actual barrier against the heat flow.

As bonding layers, either diffusion layers or cladding layers of the MCrAlY type can, in principle, be used. The most important property of the bonding layers is the ability to form the purest possible aluminum oxide on its surface, as a result of which the cyclical and static bonding of the heat insulating layer system is ensured.

MCrAlY layers contain the intermetallic β-phase NiCoAl as an aluminum reserve in a NiCoCr (“γ”) matrix. The β-phase NiCoAl, however, also has an embrittling effect, so that the Al content which can be realized in practice is 12 wt. %. To achieve a further increase in the oxidation resistance, it is possible to coat the MCrAlY layers with an Al diffusion layer by alitization. Because of the danger of embrittlement, this is limited in most cases to starting layers with a relatively low aluminum content.

To increase the Al content in the MCrAlY layer, it is proposed in German Patent Application 10 2004 045 049.8 that the alitizing be continued until an Al diffusion layer with an Al content of approximately 20% and on top of that a built-up Al layer of approximately 30% has been produced (by further alitization). The built-up Al layer contains the very brittle —NiAl phase. By means of an abrasive treatment, e.g., by blasting with hard particles, the outer built-up Al layer is removed down to the Al diffusion layer, as a result of which the Al content in the remaining diffusion layer is at least 18% and no more than 30%.

The invention is based on the task of improving the coating method of the type in question in such a way that the tendency of the internal coating to crack as a result of thermomechanical fatigue is reduced and the application of the outer coating is facilitated.

According to the invention, the task is accomplished in a method of the type in question by the characterizing features Claim 1. Advantageous embodiments of the invention are described in the subclaims. A blade produced according to the method is the object of Claim 4.

By replacing the alitized layer, which tends to be brittle, by a chromized layer as the internal coating, the more ductile Cr diffusion layer decreases the tendency for cracks to form and thus improves the so-called thermomechanical fatigue life of the highly thermally stressed blades. In addition to the improved mechanical properties, a considerable increase in the resistance to high-temperature corrosion is also achieved, which, in the case of the sulfur-contaminated cooling air, is important in the operating temperature range of the internal coating. An advantage in terms of process technology, furthermore, consists in that the alitization of the MCrAlY outer coating is made easier by the chromium enrichment at the surface achieved by the internal coating method. The duration of the alitization method can also be reduced.

An exemplary embodiment of the invention is explained in greater detail below on the basis of the drawings.

FIG. 1 shows a longitudinal section through a gas turbine blade; and

FIG. 2 shows detail Z of FIG. 1, not true to scale.

The blade 1 of a gas turbine is hollow and has cooling channels in its interior. Compressed air, which is taken from suitable stages of the compressor of the gas turbine system, flows through the cooling channels 2.

The wall of the blade 1 consists of a base material 3 of a highly heat-resistant alloy based on nickel or cobalt. The base material 3 is provided on the inner side, i.e., on the side facing the cooling channels 2, with an inner coating 4 and on the outside with an outer coating 5, the production of which is described in the following.

The starting state for the production of the coating is the finish-machined blade 1. In a first step, a bonding layer 6 of a MCrAlY alloy is applied to the outside of the blade 1 to serve as the bottom layer of the outer coating 5. Application is carried out by the low-pressure plasma spraying (LPPS) process or by the high-velocity oxygen fuel (HVOF) thermal spray process. In this way, a very dense and high-quality layer is obtained.

In the second step, the entire blade 1 is chromized by chemical vapor deposition (CVD). In this method, a hot, reactive Cr-containing gas is brought into contact with the blade 1. At elevated temperature, this gas brings about the inward-diffusion of Cr on the base material 3 and the MCrAlY bonding layer 6 as well as an outward diffusion of Ni. During this step, a Cr diffusion layer 7 forms as an intermediate layer on the bonding layer 6 of the outer coating 5 and simultaneously as an internal coating 4.

In the third step, the bonding layer 6 coated with the Cr diffusion layer 7 is subjected to alitization by chemical vapor deposition, in that the bonding layer 6 is exposed to a hot, reactive Al-containing gas. In this variant of further alitization, as described in the previously mentioned German Patent Application 10 2004 045 049.8, a built-up Al layer 8 is produced on top of the Al diffusion layer itself, this built-up layer containing the very brittle —NiAl phase.

To optimize the material properties of the base material 3, the fourth step is preferably a heat treatment of the blade 1 in the form of a solution treatment followed by age-hardening.

In the fifth step, as described in the previously cited German Patent Application 10 2004 045 049.8, the very brittle —NiAl phase is removed by an abrasive treatment, e.g., by blasting with hard particles, so that only the actual Al diffusion layer 8 remains behind.

In the sixth and last step, a heat-insulating layer 9 is applied to the chromized and alitized MCrAlY bonding layer 6 by electron beam-physical vapor deposition (EB-PVD). The heat-insulating layer 9 consists of zirconium oxide, partially stabilized with approximately 7% yttrium oxide.

Aside from the method of further alitization, the individual coating processes cited above are generally known in coating technology, so that there is no need to describe them in detail. Performing the individual method steps correctly is crucial to the success of the inventive method. 

1.-4. (canceled)
 5. A method for coating a hollow, internally cooled blade of a gas turbine, comprising: applying onto an outside surface of a base material of the blade an outer coating comprising a MCrAlY-based bonding layer; applying, by chemical vapor deposition, an inner coating consisting of a first Cr diffusion layer to an inside surface of the blade; applying a second Cr diffusion layer to the MCrAlY-based bonding layer of the outer coating, wherein the steps of applying the first Cr diffusion layer and applying the second Cr diffusion layer are performed simultaneously; applying an Al diffusion layer and an outer brittle built-up Al layer by chemical vapor deposition to the bonding layer coated with the second Cr diffusion layer; removing the outer brittle built-up Al layer by an abrasive treatment; and applying, after the step of removing, a ceramic heat-insulating layer of zirconium to the Al diffusion layer.
 6. The method according to claim 5, further comprising: subjecting the blade to a solution heat treatment before removing the outer brittle built-up Al layer by the abrasive treatment abrasive.
 7. The method according to claim 5, wherein the bonding layer is applied as a cladding layer by plasma or high-velocity spraying.
 8. The method according to claim 6, wherein the bonding layer is applied as a cladding layer by plasma or high-velocity spraying.
 9. A hollow, cooled blade of a gas turbine, comprising: an outer coating comprising a MCrAlY-based bonding layer and a ceramic heat-insulating layer of zirconium oxide applied to an outside surface of the blade on a base material of the blade; an inner coating consisting of a first Cr diffusion layer applied to an inside surface of the blade; a second Cr diffusion layer and an Al diffusion layer arranged in the outer coating; wherein the ceramic heat-insulating layer is applied to the MCrAlY-based bonding layer of the outer coating. 